I recommend that you buy an external USB drive and back up your system to it (with Time Machine or Carbon Copy Cloner) so you can get back to where you started in case of a problem.
Then, install Snow Leopard, but don't let it update to 10.6.8 if you have any NTSC-601 media in the Media 100i codec. After that, you can run the Media 100 Suite installer.

You should also make sure you install QuickTime 7 again - the default with Snow Leopard is QuickTime X which has its own issues (or is it just me?). QT 7 should be in the 'Optional Software Installation" folder (or whatever it's called) on the Snow Leopard disc.